1. Field of the Invention
The present invention refers to a charge-pump type, voltage-boosting device with reduced ripple, in particular for non-volatile flash memories.
2. Description of the Related Art
As is known, in non-volatile flash memories the part of design that concerns charge pumps plays a fundamental role. In fact, these devices are necessary in the various operating steps of the memory (programming, erasing, reading, verifying). The design step of a charge pump consists in determining the number of stages and in their sizing to obtain a maximum deliverable current as well as the desired output voltage Vout, which, in a per se known manner, in the case of positive charge pumps is:Vout=Vdd*(n+1),where Vdd is the supply voltage and n is the number of stages of the charge pump.
Normally, a flash memory has a supply voltage that varies within a given range, defined by the specifications. Consequently, in current memories, a system is provided for ON/OFF regulation of the output voltage, wherein a portion of the output voltage Vout is compared with a fixed reference voltage value. When the output voltage Vout exceeds the fixed reference voltage, no more clock pulses are sent to the pump, thus bringing about the pump arrest. As soon as the output voltage Vout drops below the desired value, the pump is started again.
The output voltage Vout obtained with the regulation method described has a ripple, which, in the case of pumps formed by the series-connection of many stages, can even arrive at some volts peak-to-peak, altering the control on the voltage to be regulated. For particular applications, such as multilevel flash memories, this represents a problem, since the correct operation of the memory (in reading, programming, and verifying) requires a sufficient precision of the supply. Furthermore, if the charge pump supplies a voltage regulator of a linear type, this must have a high power-supply rejection ratio (PSRR).
Charge pumps are known formed by a plurality of stages series-connected, which can be differently controlled according to the operating conditions. For example, U.S. Pat. No. 6,486,728 describes a multistage charge pump wherein an operational amplifier compares a portion of the output voltage with a reference voltage, and its output controls the frequency of an oscillator that generates the clock signal supplied to the stages. Furthermore, a programmed logic enables turning-on of only some of the stages.
U.S. Pat. No. 5,717,581 describes a multistage charge pump equipped with a feedback control circuit that controls conductivity of the switches included in the stages.
US-A-2002/0003448 describes a structure that is able to adapt the number of stages to be inserted according to the current and the output voltage to improve efficiency as regards consumption levels. To this end, the current and/or voltage output signal, after being converted into digital form, is supplied to a up-counter or down-counter, which, through a counting operation, determines activation or deactivation of the individual stages of the pump.
These systems do not tackle the problem of reducing the output ripple; moreover they are suited only for specific types of charge pumps and hence are not of general application.